1. Field of the Invention
The present invention relates to an electrolytic refining method for gallium and an apparatus for use in the method.
2. Description of the Related Art
Recently, the demand for metallic gallium is increasing because of its use as a raw material for GaAs, GaP, and the like, which are used as compound semiconductor devices and light emitting devices. Gallium is mainly produced as a by-product in a process for producing alumina or for smelting zinc, but, in addition thereto, scraps of semiconductor materials are also available as a raw gallium material.
As methods for refining gallium from such raw gallium materials (i.e., metallic gallium accompanied by impurities), usually well-known are the crystallization refining method, the lifting of single crystals, and the electrolytic refining method.
The crystallization refining method is a method for obtaining solid gallium, which comprises incorporating a seed crystal into the cooling medium during the solidification of the melted raw gallium material, thereby allowing the seed crystal to grow by the cooling effect exerted by the cooling medium and obtaining the refined solid gallium in the thus grown crystalline side. For instance, in Japanese Patent Laid-Open No. 50926/1990 is disclosed a crystallization refining method comprising performing the crystal growth above in multiple steps.
The method of lifting a single crystal is a refining method which comprises bringing the front end of a seed crystal in contact with a melted raw gallium material, and then slowly pulling up impurity-free single crystals having been grown from the seed crystal. For instance, Japanese Patent Laid-Open No. 243727/1990 teaches that the efficiency of refining is improved by forming an acidic solution layer on the surface of melted gallium.
The electrolytic refining method comprises performing electrolysis in an electrolytic solution using a raw gallium material as an anode. In this case, gallium and metals that are electrochemically more basic than gallium elute into the electrolytic solution, while metals that are electrochemically more precious than gallium electrolytically deposit on a cathode together with gallium. Thus, refined metallic gallium can be obtained on the cathode. For example, Japanese Patent Laid-Open No. 192877/1994 discloses a method comprising placing a melted raw gallium material on the bottom of an electrolytic cell and then performing electrolysis between the melted raw material used as an anode and a rod-like cathode. In this case, the metallic gallium deposited on the surface of the cathode drops down in the form of drops and is collected in a receptor provided on the lower side, while impurities such as indium, copper, and lead remain on the anode side.
In the crystallization refining method, the purity of gallium can not be increased unless otherwise repeating the operation. Moreover, because the process is complicated and the productivity is low, in many cases the application of this method is limited to the refinement in a high-purity region, i.e., the method is applied to the use of metallic gallium having a purity of 5N (99.999%) or higher as a raw material in order to obtain a product with a higher purity of 6N or 7N (99.9999% or 99.99999%) or even higher. That is, the method is not suitable for those having a purity of about 2N or 3N because the yield is too low. Also, concerning the method of lifting a single crystal, its application is limited to that in a high-purity region, and furthermore, it has a disadvantage that the facilities are expensive.
In contrast, the electrolytic refining method is simple as compared with the above-described two methods, does not substantially require manual operations and is inexpensive in terms of apparatus. Thus, this method is advantageous in that it is applicable as a low-purity refining method to be performed as a step until the high-purity refining method (i.e., as a pretreatment). However, the related art electrolytic refining method comprises concentrating indium, copper, lead, etc. in an anode and leaving them. Thus, if a predetermined or more amount of impurities are concentrated in the anode, the impurities are incorporated into an electrolytic solution, resulting in lowering the purity of gallium deposited on a cathode. Thus, in case where the purity of the refined gallium is regulated, the electrolysis life is automatically determined. Furthermore, the related art electrolytic refining method involves a problem that "gold " contained in semiconductor scraps, etc. can not be removed therefrom.